1. Field of the Invention
The present invention relates to a semiconductor device, such as a thin film transistor, which is appropriately used in a liquid crystal display, an image sensor, a solar cell, etc. The present invention also relates to a method for producing the same and to a method for producing a liquid crystal display apparatus utilizing such a semiconductor device.
2. Description of the Related Art
Recently, when producing a liquid crystal display, an image sensor and the like, there is a need for producing a semiconductor device such as a thin film transistor on a transparent insulating substrate so that the driving circuit, which is to be externally mounted in prior art, is mounted on the same substrate as the liquid display, the image sensor, etc. Accordingly, various production techniques are being developed.
For example, when producing a thin film transistor (TFT) on a substrate, a production method including the step of implanting impurity ions to a semiconductor film which becomes a device formation region (active region) while using the gate electrode as a mask, thereby self-aligningly forming an n-type or p-type source/drain region, is useful because a channel length of the TFT to be formed can readily be shortened for higher levels of performance. Accordingly, its development is in progress.
In order to realize lower production cost, it is preferable to use an inexpensive glass plate as a substrate. For that purpose, it is necessary that the process temperature typically be about 600xc2x0 C. (preferably about 500xc2x0 C. or less) when heat resistivity of the glass substrate is under consideration. However, it is difficult to form a silicon semiconductor film of excellent quality at such a low temperature.
For example, if a polycrystalline silicon semiconductor film is to be formed at a low temperature of about 600xc2x0 C. or less, it is generally preferable that an amorphous silicon semiconductor film is first grown on a substrate. Then, the amorphous silicon semiconductor film is annealed for crystallization (solid-phase growth) so as to obtain the polycrystalline silicon semiconductor film. The reason is that the polycrystalline silicon semiconductor film having a large crystalline granular radius and excellent characteristics can be obtained.
However, in order to have the crystallization mentioned in the above process, it is generally necessary to perform annealing at about 600xc2x0 C. for about 12 hours (preferably for about 24 hours). Heat treatment for longer hours is also required for a semiconductor device having higher field effect mobility or higher reliability.
However, longer hours of annealing results in a low throughput. Moreover, if annealing is performed at high annealing temperature for long hours, then the glass substrate may experience irreversible thermal contraction or warping. In order to resolve this situation, it is necessary to cope with two problems, one of which is to lower the annealing temperature and the other of which is to shorten the processing hours.
Described in Japanese Laid-Open Patent Publication No. 6-244103 as one of the solutions to the above-mentioned problem is a method where, by using a catalyst such as nickel (Ni) or the like, the crystallization of the amorphous silicon semiconductor film is performed at a temperature lower than the typical crystallization temperature. However, it is also described in the publication that the Ni atoms (catalyst) themselves are not desirable in the silicon film which is present as a semiconductor material. For this reason, in order to guarantee high quality of the crystalline silicon semiconductor film and high performance of the semiconductor device such as TFT which is formed of such a crystalline silicon semiconductor film, it is necessary to remove the Ni atoms (catalyst) which were added and used in the crystallization step.
Alternatively, in the case where such step of removing the catalyst element is not performed, it is necessary to control a concentration of catalyst element to be introduced to such a degree that the concentration is sufficient for prompting the crystallization of the amorphous silicon semiconductor film and, at the same time, the catalyst element which remains in the crystalline silicon semiconductor film does not have a negative effect on the characteristics of the crystalline silicon semiconductor film. However, the concentration of the catalyst element satisfying such requirements is typically very small. Accordingly, it is extremely difficult to realize the step of introducing the catalyst element while accurately controlling its concentration.
In addition to the above-described problem, in order to lower the production cost for a liquid crystal display or the like, it is strongly desired that the number of masks for photolithography needed during production be reduced, thereby simplifying the production steps. If the number of masks is reduced, then the production steps are simplified and a throughput and a production yield improve, thereby greatly reducing the production cost. However, the production method described in the conventional art requires more number of masks than generally needed, and therefore the above-mentioned objective, i.e., the simplification of the production steps, becomes difficult to achieve.
The semiconductor device of this invention includes a substrate; a line formed on said substrate; and a crystalline semiconductor film containing silicon connected to said line; wherein said crystalline semiconductor film is crystallized by annealing where a constituting material of said line functions as a catalyst.
In one embodiment of the invention, the device further includes an insulating film formed on said line, wherein said crystalline semiconductor film is connected to said line through a contact hole formed in said insulating film.
In another embodiment of the invention, the line is formed of a single layer film or of a multi-layer film made of a constituting material containing at least one material selected from the group consisting of nickel, iron, cobalt and platinum.
According to another aspect of the invention, a thin film transistor is provided. The thin film transistor includes a substrate; a line formed on said substrate; a crystalline semiconductor film containing silicon connected to said line; a gate insulating film formed on said crystalline semiconductor film; and a gate electrode formed on said gate insulating film; wherein said crystalline semiconductor film is crystallized by annealing where a constituting material of said line functions as a catalyst.
In one embodiment of the invention, the thin film transistor includes an insulating film formed on said line, wherein said crystalline semiconductor film is connected to said line through a contact hole formed in said insulating film.
In another embodiment of the invention, said crystalline semiconductor film includes a source region and a drain region; and said source region and said drain region are selectively doped with a group III element or a group V element as an impurity.
In still another embodiment of the invention, said impurity is doped self-aligningly with said gate electrode being used as a mask.
In still another embodiment, the thin film transistor further includes a shielding film formed on said substrate in the same step as for said line.
In still another embodiment, said line is formed of a single layer film or of a multi-layer film made of a constituting material containing at least one material selected from the group consisting of nickel, iron, cobalt and platinum.
According to still another aspect of the invention, a liquid crystal display apparatus is provided. The divide includes one of the above thin film transistors.
According to still another aspect of the invention, a method for producing a thin film transistor is provided. The method includes the steps of: forming a line on a substrate; forming a semiconductor film containing silicon and including an amorphous portion so as to be connected to said line; performing annealing for crystallizing said semiconductor film including said amorphous portion with a constituting material of said line being used as a catalyst, so as to obtain a crystalline semiconductor film containing silicon; forming a gate insulating film on said crystalline semiconductor film; and forming a gate electrode on said gate insulating film.
In one embodiment of the invention, the method further comprising: forming an insulating film on said line; and forming a contact hole in said insulating film, and connecting said crystalline semiconductor film to said line through said contact hole.
In another embodiment of the invention, the method further comprising the step of selectively doping a predetermined region of said crystalline semiconductor film with a group III element or a group V element as an impurity so as to form a source region and a drain region.
In still another embodiment of the invention, said impurity is doped self-aligningly with said gate electrode being used as a mask.
In still another embodiment of the invention, a shielding film is further formed on said substrate in the step of forming said line.
In still another embodiment of the invention, a process temperature for the annealing for crystallization is lower than the crystallization temperature for amorphous silicon by about 20xc2x0 C. to about 150xc2x0 C.
In still another embodiment of the invention, the line is formed of a single layer film or of a multi-layer film made of a constituting material containing at least one material selected from the group consisting of nickel, iron, cobalt and platinum.
According to still another aspect of the invention, a method for producing a liquid crystal display apparatus is provided. The method includes one of the methods for producing thin film transistors cited above.
Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor device or a thin film transistor having excellent mass producibility and low production cost, yet having excellent operational characteristics, and a method for producing the same, and of (2) a liquid crystal display apparatus using such devices and having low production cost yet having high display definition, and a method for producing the same.
Hereinafter, the functions of the present invention will be described.
Because of the above-described structure, the semiconductor device or the thin film transistor according to the present invention can be readily produced in a fewer number of production steps. For example, the number of photolithographic masks needed for the production becomes small.
Furthermore, because of the lines formed of a single layer film or a multi-layer film made of a material containing at least one of nickel, iron, cobalt and platinum, the crystallization proceeds with the above-mentioned material functioning as a catalyst, starting with the portion which is in contact with the lines, and the semiconductor film having excellent crystallinity can be obtained in the annealing for the crystallization of the silicon-containing semiconductor film including amorphous portion. Accordingly, as described in Japanese Laid-Open Patent Publication No. 6-244103, the silicon-containing semiconduct or film having excellent crystallinity can be obtained by annealing at a temperature lower than the typical crystallization temperature for amorphous silicon by about 20xc2x0 C. to about 150xc2x0 C. If the annealing temperature can be lowered, then an inexpensive glass substrate having low heat resistance can be used.
Furthermore, in the above-described structure, the catalyst material for the crystallization also serves as a constituting material of the lines. Therefore, not like the case where a thin film for introducing the catalyst element is provided, it is not necessary to perform the step for removing the portion including the catalyst after the production of the semiconductor apparatus or the thin film transistor. Accordingly, the production steps can be simplified. Moreover, fine control of the concentration of introduced catalyst is not required, and excellent uniformity of devices to be formed as well as excellent repeatability can be achieved. The device is thus suited for mass production.
The bulk resistivities for nickel, iron, cobalt and platinum are about 7.2 xcexcxcexa9xc2x7cm, about 9.8 xcexcxcexa9xc2x7cm, about 6.4 xcexcxcexa9xc2x7cm and about 10.6 xcexcxcexa9xc2x7cm, respectively. These values are lower than those for Ta (about 15 xcexcxcexa9xc2x7cm) or Cr (about 17 xcexcxcexa9xc2x7cm), which are often used as a material for the lines. Therefore, the low resistivity of lines can be realized.
If the shielding film is simultaneously formed when the lines are formed on the substrate, the thin film transistor having excellent light resistivity can be obtained without increasing the number of production steps.
Furthermore, by using the thin film transistor which has the simple structure as described above as well as the excellent crystallinity and operational characteristics, the liquid crystal display apparatus which has high display definition can be easily and inexpensively obtained.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.